How many iron(ii) ions combine with oxygen to form iron(ii) oxide answer?

Iron (II) means that Fe has lost two electron i.e Fe⁺².

We also know that Oxygen normally exists in -2 oxidation state i.e. O⁻².

And, for a neutral Iron (II) Oxide the overall charge on it must be zero.

So,
                                    + 2 + (-2)  =  0
Means,
           For one atom of Fe⁺² we need one atom of O⁻².

                                                FeO

Result:
           One Iron (II) ion combines with oxygen to form iron(ii) oxide.